Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Nomeie o número de países que possuem o dólar americano.
 ###Tabelas: CREATE TABLE controversy (controversy_id AUTO_INCREMENT PRIMARY KEY, Category VARCHAR, Frequency VARCHAR, White VARCHAR, Brown VARCHAR, Black VARCHAR, Amerindian VARCHAR, Yellow VARCHAR, Total VARCHAR, difference between White and Black INT) CREATE TABLE pounds_as_a_measure_of_cannon_bore (pounds_as_a_measure_of_cannon_bore_id AUTO_INCREMENT PRIMARY KEY, gun class (pdr.) INT, shot diameter (cm) INT, shot volume (cm 3 ) INT, approx. service bore (cm) INT, mass of projectile (kg) INT) CREATE TABLE runners-up_6 (runners-up_6_id AUTO_INCREMENT PRIMARY KEY, Date VARCHAR, Tournament VARCHAR, Surface VARCHAR, Partnering VARCHAR, Opponent in the final VARCHAR, Score VARCHAR) CREATE TABLE main_issuers (main_issuers_id AUTO_INCREMENT PRIMARY KEY, Currency VARCHAR, Country VARCHAR, Generic Name or Nickname VARCHAR, Rating ( S&P / Moodys ) VARCHAR, Negotiable debt at mid-2005 ( US dollar bn equivalent) INT, Government financial liabilities as % of GDP (end 2003 - source : OECD ) VARCHAR, Issuer VARCHAR, Internet site VARCHAR) CREATE TABLE cover_version (cover_version_id AUTO_INCREMENT PRIMARY KEY, Album VARCHAR, Release date VARCHAR, Label VARCHAR, Format VARCHAR, Covered Song VARCHAR, Original Song VARCHAR)  
SELECT COUNT Country FROM main_issuers WHERE Currency = US dollar